1. Field of the Invention
The present invention relates to a method for finish coating on polyurethane formed articles.
2. Description of the Prior Art
Polyurethane formed articles have found wide applications in various fields, in particular for parts of automobiles owing to their features of excellence in the moldability, resiliency, toughness and so on. In coating polyurethane formed articles with coating compositions for decorative purpose, a special primer coating assigned exclusively for polyurethane resin has been employed, since the adhesion of usual finish coatings onto polyurethane substrates is in general insufficient. Thus, in the conventional technique for coating polyurethane formed articles, a twice-baking method is incorporated in such a manner that the polyurethane formed article is pretreated by vapor degreasing with an organic solvent, such as, 1,1,1-trichloroethane or 1,1,2-trichlorotrifluoroethane, or by alkali-degreasing and the so pretreated article is subjected to a primer coating, followed then by heat drying, whereupon a finish coating is applied with subsequent another heat drying.
As the primer coating for polyurethane resin formed articles, there has heretofore been employed a specific primer coating composition exclusive for polyurethane resin which reveals superior adhesion onto the polyurethane substrate, such as for example, an undercoat composition for an elastomer skin coating based on a reaction product of a polycarboxylic acid or its anhydride with a hydroxyl-containing urethane reaction product (U.S. Pat. No. 3,962,369 and the corresponding Brit. Pat. No. 1,458,091 and Japanese Patent Publication No. 36285/1976) or a primer coating composition using an aliphatic linear polyurethane and N-methyl-pyrrolidone (Catalogue RR 2566/1 of the firm Bayer). After the application of such a primer coating composition, a heat drying is by all means required before the application of the skim coating, in order to avoid any defective influence upon the skin coating.
As to the skin coating composition for application on polyurethane formed articles, there have been found in the practical use, for instance, skin coating compositions containing, as the essential resin component, a polyester resin or an acrylic resin together with an aminoplast or an isocyanate.
The conventional technique for coating polyurethane resin formed articles requires each separate procedure for the primer coating and skin coating with two heat drying procedures and, thus, two coating compositions are necessary with corresponding complicated procedures and installations, neccesitating longer coating time and higher coating costs.
Though the primer coating exhibits in itself better adhesion onto the polyurethane substrate, it is inferior in the weather resistance, chemical resistance, appearance and so on. The skin coating is superior in itself in the weather resistance and chemical resistance, but is inferior in the adhesion onto the polyurethane substrate. Thus, there was a practical demand for a single coating composition which reveals not only a better adhesion onto the polyurethane substrate comparable to the primer coating of the prior art but also excellent in the weather resistance, chemical resistance, appearance and so on comparable to the skin coating of the prior art.